The proper function of the nervous system is critically dependent on the formation of highly specific connections during embryonic development. In many systems, developing neurons make functionally appropriate axonal projections and synaptic connections after first adopting unique identities through an enigmatic process known as cell fate specification. The long range goal of this research program is to understand the cellular and molecular mechanisms that contribute to the specification of different neuronal cell fates. We are focusing on the specification of proprioceptors, the primary sensory neurons of the dorsal root ganglion (DRG) that innervate the stretch receptors of muscle. As the DRG develops, different neurons within the ganglion adopt different functional identities and make distinct axonal projections to different regions of the spinal cord based on these identities. An intriguing feature of this system is that the ultimate identity of these sensory neurons is determined by their peripheral targets. We have identified a specific signaling molecule, neurotrophin-3 (NT3), as an important muscle-derived influence that is sufficient to promote the differentiation of proprioceptive sensory neurons in vivo, even in the absence of limb muscles. Because NT3 also promotes the survival of these neurons, its precise role in specification is currently unclear. To elucidate the role of muscle-derived NT3 in the specification of proprioceptive neurons, we will pursue three related lines of evidence. First, we will determine if proprioceptive neurons already have a distinct identity before encountering their muscle targets. Second, we will determine if these neurons can differentiate in the absence of muscle-derived NT3 when their survival is ensured by viral-mediated expression of cell death repressors. Third, we will transplant identified proprioceptive neurons to determine if they become irrevocably committed to this identity by interacting with their muscle targets. These experiments will provide important new insights into both the timing and mechanisms of sensory neuron specification. They will allow us to establish if muscle-derived NT3 instructs the differentiation of proprioceptive neurons or if it serves only as a survival factor for neurons that have already been specified to adopt this fate. Because these experiments will determine if neurotrophins can influence the fundamental identity of post-mitotic sensory neurons, this project has important implications for the potential therapeutic use of these powerful signaling molecules.